The contrast button is disabled when the 540c is set to 16 bit color. This is normal behavior. The reason that contrast is disabled is because all of the available video memory is used to support thousands of colors and there is no additional memory available to support contrast.
Contrast on the LCD of the 540c uses an off screen color table. Users cycle through the off screen table when they adjust contrast. Since there is no memory to create the off screen table the contrast can not be adjusted.
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